Etching solution and process



Patented Sept. 22,1953

ETCHING SOLUTION AND PROCESS Richard H. Wynne, Jr., Pittsburgh, Pa.,assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., acorporation of Pennsylvania No Drawing. Application August 9,1952,Serial No. 303,610

. 6 Claims. 1 i This invention relates to a novel etching solution andprocess for preparing the surface of metals and metalloids such asgermanium and silicon for use in semiconductor devices such asrectifiers and transistors.

In such devices, the most desirable characteristics are obtained only ifthe germanium surface is given a suitable preparatory treatment. Thistreatment involves cutting a section, as for instance a wafer, from aningot of germanium, grinding the surface to a dull gray finish with asuitable wet abrasive powder, washing and drying the surface, and thenetching the surface in a suitable etchant solution until the dull,lustreless gray appearance has been replaced by a surface having asharply delineated grain structure and a metallic lustre on the grainsurfaces. Suitable electrodes are then applied to the treated surface.The device is then subjected to an electrical forming operation wellknown to those skilled in the art.

The etchant solution used in this treatment has a considerable effectupon the characteristics of the finished device. Prior to thisinvention, an etchant solution commonly used was an aqueous solution ofnitric acid, hydrofluoric acid and copper nitrate. In my experiments, Ihave discovered an etching process and solution that provides an etch onthe surface of metal and metalloid bodies, such as germanium andsilicon, which is far superior to that accomplished by the solutiondescribed above, as will be hereinafter shown.

Accordingly, an object of this invention is to provide an improvedprocess for preparing the surlace of a body of germanium for use in asemiconductor device by etching said surface in an aqueous solution ofhydrofluoric acid, nitric acid, and silver nitrate.

A further object of this invention is to provide an improved etchingsolution, the essential ingredients of which comprises an aqueousmixture of hydrofluoric acid, nitric acid, and silver nitrate.

The aqueous solution of hydrofluoric acid, nitric acid and silvernitrate may be used effectively for etching germanium in a considerablerange of proportions and dilutions. The etching solution of thisinvention comprises the following range of ingredients by weight; from50% to 86% by weight of water and from 14% to 50% by weight of acid, theacids being present in ratios of hydrofluoric acid (HF) to nitric acid(HNO3) of from 6:1 to 1:16 by weight, and silver nitrate being dissolvedin the aqueous acid solution in an amount of at least 1% of the weightthereof.

To each of the above three solutions was added 1y delineated grainboundaries.

an aqueous solution of silver nitrate to provide 0.200 g. of silvernitrate per 10 ml. total volume of aqueous acid solution.

A germanium body upon being immersed for one minute in any of the aboveetching solutions had a brilliant specular surface with sharp- Whenviewed by reflected light, some grains appeared light while othersappeared dark. When the angle of the surface or the angle of theincident light was changed, the bright and dark areas were reversed.This characteristic is termed metallic lustre.

The extent to which the total mixed acid volume, as received from thesupplier, may be diluted'with water ranges from 8 volumes acid: 2volumes water to 5 volumes acid: 20 or more volumes water. The etchingtime required increases with the dilution. The practical limitation ofdilution is reached when silver deposited on the gernanium surface failsto redissolve in the mixed aci s.

The silver nitrate may range from its maximum solubility in the dilutedmixed acids down to approximately 0.10 g. in a total volume of 10 ml.,such last concentration being approximately 1% by weight. Below thisvalue, especially at approximately 0.05 g. and less per 10 ml. of amixed acid solution, the depth of etching becomes too great even in ashort time of 15 seconds.

The ratio of concentrated hydrofluoric acid to nitric acid by volume, asreceived from the supplier, may be varied from 9:1 to 1:9 if dilutedwith the proper amount of water to keep the action at a reasonablespeed, in other words, to produce the desired etched surface in one minue. 7

Immediately upon removal from the etching solution, the etched germaniumbody is in a running stream of cold water and then drying in an airblast.

It was found that the germanium bodies etched in the above manner gave amuch sharper etch than the germanium bodies etched in the conventionalhydrofluoric acid, nitric acid and copper nitrate solutions. With thesilver nitrate as an additive, the crystal surface of the germanium bodypresented a. number of facets all ofv which give Miller indices of(111). This Was not true with the conventional etching solution hithertoknown, having copper nitrate as an additive. The copper nitrate etchantgave Miller indices of (111) only when the surface being etched wasapproximately a (111) surface prior to immersion.

llt has also been discovered that a germanium body etched with asolution having copper nitrate as an additive causes a far greater rateof decay of minority carriers than does a germanium body etched with asolution having silver as an additive. This means that when holes are injected into an N-type germanium sample, they will last much longer if asolution with silver nitrate as an additive has been used, than theywould if copper nitrate had been used. Thus, it follows that an emitterin a transistor will be more efiicient when a silver nitrate etchant isused than when a copper nitrate etchant is used.

Further, it has been found that germanium etched with a solution havingsilver nitrate as an additiv provides a microscopic roughness on thesurface of the germanium body which is not obtained with an etchingsolution having copper nitrate as an additive. With this added degree ofroughness, contact probes used in making up a transistor device do nothave a tendency to slide on the surface of the treated germanium body.

While the etching solutions and process disclosed herein areparticularly directed to treating germanium, it is to b understood thatthey are equally applicable in treating silicon.

The description of the invention above is intended to be illustrative ofand not in limitation on the scope of the invention.

What is claimed is:

1. The process of preparing the surface of a body of germanium for usein a semiconductor device which comprises etching said surface in anaqueous solution of hydrofluoric acid, nitric acid, and silver nitrate,and then Washing said etching solution from said surface.

2. The process of claim 1 wherein the proportions of ingredients presentin 100 parts by weight of etching solution comprises a mixture of fromto 86% by weight of water and from 14% to 50% by weight of acids, theacids being present in ratios of HF to HNOs of from 6:1 to 1:16 byweight, and silver nitrate being dissolved in the solution in an amountof at least 1% of the weight of the aqueous acid solution.

3. A solution for etching the surface of a semiconductor body inpreparation for use in a semiconductor device in which parts by weightof the solution is an aqueous acid mixture comprising 50% to 86% byweight of water, and 14% to 50% by weight of acids, the acids beinghydrofluoric acid and nitric acid, the ratio of HF to I-INO3 varyingfrom 6:1 to 1:16 by weight and silver nitrate being dissolved in thesolution in an amount of at least 1% of the weight of the aqueous acidmixture.

4. In the process of preparing the surface of a body of germanium foruse in a semiconductor device, the surface having substantially noportion with oriented Miller indices of (111), comprising etching thesurfaces in an aqueous solution comprising a mixture of from. 50 to 86%by weight of water and from 14% to 50% by weight of acids, the acidsbeing present in ratios of HF to HNO3 of from 6:1 to 1:16 by weight, andsilver nitrate being dissolved in the solution in an amount of at least1% of the weight of the aqueous acid solution, the etching being out fora period of time of the order of 15 seconds to one minute whereby thesurface comprises a large proportion of facets having Miller indices of111), and then Washing the etching solution from said surface.

5. A germanium crystal having an etched probe surface with microscopicroughness derived by the process of claim 4.

6. A germanium crystal having an etched surface cut from the crystal atan orientation other than at 2. Miller index of (111), the etchedsurface comprising a large proportion of facets having Miller indices of(111) thereby being of microscopic roughness improving its cooperationwith contact probes.

RICHARD H. WYNNE, JR.

Name Date Lark-Horvitz July 11, 1950 Number

1. THE PROCESS OF PREPARING THE SURFACE OF A BODY OF GERMANIUM FOR USEIN A SEMICONDUCTOR DEVICE WHICH COMPRISES ETCHING SAID SURFACE IN ANAQUEOUS SOLUTION OF HYDROFLUORIC ACID, NITRIC ACID, AND SILVER NITRATE,AND THEN WASHING SAID ETCHING SOLUTION FROM SAID SURFACE.
 2. THEPRROCESS OF CLAIM 1 WHEREIN THE PROPORTIONS OF INGREDIENTS PRESENT IN100 PARTS BY WEIGHT OF ETCHING SOLUTION COMPRISES A MIXTURE OF FROM 50%TO 86% BY WEIGHT OF WATER AND FROM 14% TO 50% BY WEIGHT OF ACIDS, THEACIDS BEING PRESENT IN RATIOS OF HF TO HNO3 OF FROM 6:1 TO 1:16 BYWEIGHT, AND SILVER NITRATE BEING DISSOLVED IN THE SOLUTION IN AN AMOUNTOF AT LEAST 1% OF THE WEIGHT OF THE AQUEOUS ACID SOLUTION.